Wednesday, November 7, 2012

When writing custom passes for a compiler, it's often a good idea to try running them on real-world programs to assess things like scalability and correctness. Taking a large project and seeing your pass work (and provide useful results!) is an exhilarating feeling. On the other hand, trying to feed in your compiler options into build systems is a good route to an insane asylum.

I complained some time ago about autoconf 2.13 failing because it assumes implicit int. People rightly pointed out that newer versions of autoconf don't assume that anymore. But new versions still come with their own cornucopias of pain. Libtool, for example, believes that the best route to linking a program is to delete every compiler flag from the command line except those it knows about. Even if you explicitly specify them in LDFLAGS. Then there's this conftest program that I found while compiling gawk:

/* Define memcpy to an innocuous variant, in case <limits.h> declares memcpy.
For example, HP-UX 11i <limits.h> declares gettimeofday. */#define memcpy innocuous_memcpy/* System header to define __stub macros and hopefully few prototypes,
which can conflict with char memcpy (); below.
Prefer <limits.h> to <assert.h> if __STDC__ is defined, since
<limits.h> exists even on freestanding compilers. */#ifdef __STDC__
# include <limits.h>
#else
# include <assert.h>
#endif
#undef memcpy/* Override any GCC internal prototype to avoid an error.
Use char because int might match the return type of a GCC
builtin and then its argument prototype would still apply. */#ifdef __cplusplusextern"C"#endifchar memcpy ();
/* The GNU C library defines this for functions which it implements
to always fail with ENOSYS. Some functions are actually named
something starting with __ and the normal name is an alias. */#if defined __stub_memcpy || defined __stub___memcpy
choke me
#endifint
main ()
{
return memcpy ();
;
return0;
}

…I think this code speaks for itself in how broken it is as a test. One of the parts of the compiler pass involved asserted due to memcpy not being used in the right way, crashing the compiler. Naturally, this being autoconf, it proceeded to assume that I didn't have a memcpy and thus decided to provide me one, which causes later code to break in spectacular bad function when you realize that memcpy is effectively a #define in modern glibc. And heaven forbid if I should try to compile with -Werror in configure scripts (nearly every test program causes a compiler warning along the lines of "builtin function is horribly misused").

The saddest part of all is that, as bad as autoconf is, it appears to be the least broken configuration system out there…